Measuring viscosity has been a requirement for many industrial processes which have their beginning on a smaller scale such as in a laboratory. The several types of viscometers which are employed as control instruments are discussed below for background information.
Various instruments for controlling viscosity and consistency include viscometers such as the Saybolt which are based on timed discharge through nozzle which is commonly used for expressing viscosities of oils.
Timed fall of a ball or rise of a bubble is employed in a typical laboratory method for measuring oil viscosity.
A continuous consistency meter, such as the Fischer and Porter Company instrument, is based on a gear pump diverting a portion of product stream through a flow bridge where a pressure differential between two reference points is established. The differential pressure is a direct measure of the consistency of the materials.
Another type viscometer by Norcross Corporation employs a piston which is raised in a time sequence and falls by gravity through a liquid. Time of fall is recorded as a measure of viscosity.
A torque to rotate a torque element in a liquid is the principle employed in the Brookfield Engineering Company viscometer. In this viscometer a synchronous motor drives a vertical spindle with disk, paddle or cylinder submerged in a test liquid. Drive is through a calibrated spring. Angular lag of spindle behind motor is proportional to viscosity and is measured in various ways.
A viscosity-sensitive rotameter by Fischer and Porter Company employs rotameter bobs which are designed for either sensitivity or immunity to viscosity. With constant flow rate a sensitive bob can be calibrated for viscosity. One method is to use an immune bob to set flow at a series of index marks to measure viscosity by reference to the calibrated scale.
A viscometer which employs a friction tube outside the main stream of flow is used for a wide range of industrial liquids for remote recording and control. The pressure drop through friction tube is achieved where pressure drop across ends of tube is measured by pneumatic force-balance type differential pressure transmitter in terms of absolute viscosity. The results gives direct solution to Poiseville's equation. Thus, the liquid is pumped at constant rate through a friction tube in viscous flow to measure or determine viscosity.
The traditional methods of generating flow through a friction tube as a means for measuring viscosity described above require constant flow pumps, valves and fittings for taking a continuous sample from the process line; however, this method does not measure actual in-line conditions. Particularly, in measuring viscosity with a device which is subject to in-process fluid line conditions of pressure and temperature requires a special design. The lack of this special design has been a shortcoming of the prior art viscometers.
Therefore, an object of this invention is to provide a device which is truly an in-line viscometer which employs a friction tube.
A further object of this invention is to provide an inline viscometer, which by actually being installed in a process fluid line, is subjected to the same pressure and temperature that the fluid being measured is subjected to and the viscosity under the process conditions is thus accurately determined under the same conditions as the main stream flow conditions.